COLUMBIA  LIBRARIES  OFFSITE 

HEALTH  SCIENCES  STANDARD 


DENTISTRY. 


GEORGE    B.  SNOW, 


fiKtko 


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VULCANIZING 


'—  AS    Al'ri.lKli     TO 


DENTISTRY 


GEORGE  B.  SNOW.  d.  d.  s. 


HU  FFALO,    N.    Y., 

18  9  2. 


SSMffato,    91.    <3l|. 


Copyrighted,  1892,  by  George   B.  Snow. 


THE   VULCANIZER 

t.YULCANIZERS,  if  made  by  manufacturers  of  experience,  may 
be  depended  upon  to  be  strong  enough  to  withstand  a  pres- 
sure three  or  four  times  as  great  as  that  incident  to  the  vulcanizing 
process.  No  house,  having  any  regard  for  its  reputation,  can 
afford  to  offer  one  for  sale  unless  it  is  of  undoubted  strength. 
When  their  manufacture  is  undertaken  by  those  having  no  ex- 
perience, the  result  is  very  apt  to  be  unsatisfactory,  both  to  the 
maker  and  purchaser.  Some  of  the  patterns  in  the  market, 
though  apparently  strong,  v^^ill  yield  gradually  to  the  strains  they 
■are  subjected  to,  and  in  a  short  time  the  packnig  joint  will  get  out 
of  true,  and  it  will  then  be  difficult  to  keep  them  steam-tight. 
Dentists  are  therefore  cautioned  against  purchasing  vulcanizers  of 
new  and  untried  designs,  unless  the  maker  will  give  a  full  and 
•strong  warrantee  that  they  will  not  get  out  of  true  by  use.  When 
the  cover  of  the  vulcanizer  is  attached  to  the  pot  by  a  screw 
thread,  as  in  the  well-known  Whitney  pattern,  there  is  no  danger 
of  this  trouble.  The  caution  applies  more  especially  to  those  in 
which  the  cover  is  held  by  a  clamping  device,  which  only  has  a 
hold  upon  the  edge  of  the  pot  at  intervals. 

A  good  rule  is  not  to  buy  any  make  unless  it  is  known  to  be 
reliable,  either  by  personal  experience,  or  by  the  testimony  of 
some  one  who  has  used  one  of  the  kind  long  enough  to  develope 
its  weak  points,  if  it  has  any. 

A  vulcanizer  may  be  of  the  proper  design,  and  still  be  un- 
satisfactory to  the  user  if  it  is  not  well  fitted.  This  is  a  point  in 
which  inexperienced  makers  are  apt  to  fail,  and  constitutes  a  good 
reason  for  not  giving  their  goods  a  trial. 

It  is  now  proposed  to  touch  upon  a  tew  points  in  the  matter 
of  the  proper  care  of  vulcanizers,  and  to  show  how  and  when  they 
become  unsafe  for  use.  It  seems  to  be  the  opinion  of  some  dent- 
ists, that  a  vulcanizer  should  remain  good  indefinitely;  but  it  is 
often  the  case  that  one  returned  to  the  maker  for  supposed  triviaJ 


repairs,  is  found  to  be  in  such  condition  as  to  warrant  its  condem- 
nation. They  are  provided  with  ample  safeguards,  by  which  they 
will  be  relieved  from  over-pressure,  if  it  should  occur.  But  no 
such  appliance  can  be  made  which  cannot  be,  either  ignorantly  or 
designedly,  put  out  of  order;  neither  can  a  vulcanizer  be  made  so 
strong  that  it  will  be  safe  under  any  attainable  pressure.  It  must 
be  carefully  and  intelligently  managed  to  insure  safety. 

Vulcanizers  are  gradually  weakened  and  eventually  destroyed 
by  corrosion.      To  illustrate  the 


manner  in  which  it  takes  place, 
the  accompanying  engraving, 
representing  a  section  from  a 
condemned  vulcanizer,  is  pre- 
sented. The  original  thickness 
of  the  metal  is  fully  preserved 
at  the  bottom  of  the  pot,  and 
there  is  but  a  slight  deficiency 
at  the  mouth.  The  action  has 
been  wholly  upon  the  sides,  and 
at  the  middle  the  metal  has 
wasted  away  until  it  is  scarcely 
thicker  than  paper.  It  will  be 
seen  to  have  been  the  greatest 
at  the  water  line;  showing  that 
it  is,  in  all  probability,  the  joint 
effect  of  air  and  moisture. 

Instances  have  been  known 
of  the  use  of  vulcanizers  with- 
out accident  until  the  sides 
have  been  actually  eaten  through 
so  as  to  leak.  This  shows  how 
little  strength  is  really  re(|uired 
to  withstand  the  pressure  due  to 
the  vulcanizing  process,  if  care 
is  taken  that  it  be  not  exceeded. 

When  the  sides  of  th^  vulcanizer  are   weakened  by  corrosion,. 
the  fact  is  easily  ascertained   by    tai^i)ing  them  lightly  with  a  small 


hammer.  One  weighing  one  or  two  ounces  is  suitable  for  the  pur- 
pose. If  the  metal  is  thick  and  strong,  the  hammer  will  rebound 
from  a  light  blow.  When  the  metal  is  quite  thin,  the  sensation 
will  be  as  though  the  blow  were  delivered  upon  lead.  There  wilt 
be  little,  if  any  rebound,  and  the  metal  will  be  dented  and  driven 
in  very  easily. 

Sometimes,  though  happily  very  seldom,  corrosion  has  been 
known  to  attack  the  brazing  in  the  seams  of  the  pot.  This  usually 
takes  place  locally,  so  that  due  warning  is  given  by  the  occurrence 
of  a  leak.  No  instance  is  known  of  a  dangerous  accident  result- 
ing from  this  cause. 

Occasionally,  a  crack  runs  around  the  bottom  of  the  pot  at 
the  corner,  and  unless  it  is  noticed  in  time,  the  bottom  is  blown 
out.  This  will  not  occur  unless  the  juncture  of  the  bottom  and 
sides  is  brought  to  an  angle.  They  should  be  joined  by  an  easy 
curve;  and  in  purchasing,  this  fact  should  be  kept  in  mind. 

As  corrosion  is  the  effect  of  the  combined  influences  of  air 
and  moisture,  the  durability  of  the  vulcanizer  will  be  prolonged 
by  expelling  the  air  before  vulcanizing,  and  by  keeping  it  dry  and 
clean  when  not  in  use. 

When  a  screw  fastening,  like  that  of  the  Whitney  vulcanizer, 
is  employed,  mischief  is  often  the  result  of  the  inordinate  use  of 
black  lead  or  soapstone  powder  upon  the  packing  joint,  and  in- 
cidentally upon  the  screw.  The  particles  of  which  either  of 
these  powders  is  composed  are  quite  hard,  and  wnll  wear  away 
metal  if  placed  between  two  rubbing  surfaces.  The  screw 
threads  of  vulcanizers  are  sometimes  so  badly  worn  by  their 
action  that  they  have  not  sufficient  hold  upon  each  other  to  retain 
the  cover. 

The  reason  for  applying  either  of  these  powders  to  the  sur- 
face of  the  packing  is  often  misapprehended.  Their  only  office 
is  to  prevent  the  packing  from  sticking  to  the  edge  of  the  pot. 
Only  a  small  quantity  is  necessary  for  this  purpose,  and  it  will 
need  renewal  but  seldom.  If  it  is  applied  too  liberally  or  too 
often,  it  will  form  a  porous  coating  on  the  surface  of  the  packing, 
through  which  the  steam  will  leak.  If  the  coating  attains  much 
thickness,  it  will  scale  off  in  patches,  and  the  leakage,  which  may 


6 

have  been  almost  impeiceptible  before,  will  be  increased  so  as  to 
be  annoying.  Possibly  the  dentist  does  not  detect  the  cause  of 
the  trouble,  and  thinking  that  the  vulcanizer  "works  hard," 
applies  oil  to  the  thread.  This  becomes  gummy  and  dry  by  the 
heat  of  vulcanizing,  and  the  cover  is  virtually  cemented  to  the 
pot.  It  is  now  removed  only  with  great  difficulty.  As  a  rule, 
when  the  packing  of  a  vulcanizer  is  in  good  order  and  steam- 
tight,  the  less  that  is  done  to  it,  the  better. 

Sufficient  steam  room  should  always  be  left  in  the  vulcanizer 
when  it  is  closed.  If  it  is  filled  with  water,  and  room  is  not 
allowed  for  its  expansion  when  heated,  a  pressure  will  be  developed 
much  greater  than  that  due  to  the  production  of  steam.  The 
safety  disk  may  be  blown  out  in  this  instance  when  the  thermome- 
ter only  indicates  280''  or  300°;  or  if  the  safety  apparatus  be  put 
out  of  order,  the  vulcanizer  pot  may  be  bulged  and  stretched  out 
of  shape,  or  a  rupture  may  result.  Water  is  inelastic,  and  when 
it  is  closely  confined,  its  expansion  when  heated  will  generate  a 
force  which  is  practically  irresistible.  It  is  an  easy  matter,  if  a 
vulcanizer  is  completely  filled  with  water,  to  obtain  a  pressure  of 
even  a  thousand  pounds  to  the  inch  without  heating  it  to  the 
boiling  point,  if  it  should  be  able  to  withstand  the  strain  without 
yielding. 

The  management  of  the  vulcanizer  is  a  matter  which  should 
be  thoroughly  understood.  If  it  is  intrusted  to  a  careless  boy 
who  has  had  no  proper  instruction,  there  is  a  strong  probability  of 
an  accident  occurring,  sooner  or  later. 


THE    THERMOMETER. 

sV^  l/rHOUGH  the  thermometer  is  almost  universally  employed 
for  indicating  the  temperature  of  the  vulcanizer,  the  pecu- 
liarities of  its  action  as  so  applied  have  been  but  little  studied. 
Dentists  put  the  utmost  faith  in  its  indications,  when,  in  fact,  it  is 
only  under  certain  conditions  that  it  shows  the  exact  temperature 
of  the  vulcanizer  and,  as  will  be  seen,  these  conditions  do  not 
usually  obtain. 


When  thermometers  were  first  used  with  vulcanizers,  they 
were  so  mounted  that  their  bulbs  projected  into  the  interior  of  the 
vulcanizer,  and  were  in  direct  contact  with  the  steam.  The  trans- 
mission of  heat  was  thus  direct,  and  the  true  temperature  of  the 
interior  of  the  vulcanizer  was  registered,  provided  the  air  was  ex- 
pelled therefrom;  a  matter  which  will  be  explained  later  on. 

It  was  soon  found  that  the  steam  exerted  a  solvent  action 
upon  the  glass,  and  that  the  bulb  became  opaque;  fine  cracks 
appearing  upon  its  surface,  which  finally  extended  through  the 
thickness  of  the  glass,  causing  the    destruction  of  the  instrument. 

To  obviate  this  objection,  and  to  increase  the  durability  of 
the  thermometer,  it  was  set  in  a  mercury  bath,  an  invention  of  the 
late  Dr.  Geo.  E.  Hayes.  A  cupped  nipple,  threaded  on  its  exter- 
ior, was  formed  upon  the  top  of  the  vulcanizer,  upon  which  the 
thermometer  case  was  screwed ;  the  bulb  dipping  into  the  cup, 
which  contained  a  little  mercury.  Peifect  metallic  connection  was 
thus  made  between  the  vulcanizer  and  thermometer,  and  heat  was 
promptly  transmitted  from  the  one  to  the  other;  while,  as  the  ther- 
mometer was  no  longer  subjected  to  the  destructive  action  of  the 
steam,  there  was  nothing  to  destroy  it  but  accident  or  bad  usage. 
The  employment  of  the  mercury  bath  with  dental  vulcanizers  has 
therefore  become  almost  universal. 

The  mercury  bath  was  first  employed  upon  a  style  of  vulcan- 
izer now  obsolete,  known  as  the  Hayes  Oven,  which  had  a  petticoat 
jacket,  which  carried  the  escaping  heat  from  the  lamp  over  the  top 
of  the  vulcanizer,  and  threw  a  current  of  hot  air  against  the  bot- 
tom of  the  thermometer  case.  This  counteracted  the  effects  of 
radiation  so  perfectly  that  experiment  showed  the  indication  of  the 
mercury  bath  thermometer  to  be  identical  with  that  of  one  project- 
ing into  the  steam  space.  When  the  mercury  bath  was  first  applied 
to  vulcanizers  of  the  ordinary  pattern,  the  difference  in  the  con- 
ditions was  not  noticed,  and  it  was  assumed  that  the  indica- 
tions of  the  thermometer  were  correct. 

When  Gas  Regulators  came  into  use,  it  was  noticed  that  there 
was  a  variation  at  times,  either  in  the  action  of  the  regulator  or  of 
the  thermometer  used  in  connection  with  it.  When  the  regulator 
was  tested  with  a  steam  gauge,  however,  the  two  agreed;  showing 


that  the  thermometer  was  at  fault.  To  ascertain,  if  possible, 
where  the  trouble  really  lay,  a  vulcanizer  was  prepared,  and 
mounted  with  three  thermometers,  which  had  been  tested  as  to 
their  accuracy.  One,  (No.  i,  see  diagram,)  was  of  extra  length, 
and  extended  into  the  vulcanizer  so  far  that  its  bulb  was  below 
the  water  line.  No.  2  was  set  in  the  old  way  as  a  "steam  "  ther- 
mometer; its  bulb  projecting  into  the  steam  space.  The  third  was 
set  in  a  mercury  bath,  in  the  ordinary  manner.  A  gas  regulator, 
steam-gauge,  and  blow-off  valve  were  also  attached  to  the  -Vulcan- 
izer. 

When  heat  was  supplied,  it  was  found  that  no  two  of  the  ther- 
mometers registered  alike,  nor  did  either  of  them  agree  with  the 
published  tables  of  steam  pressures  and  temperatures  and  the 
readings  of  the  steam  gauge.  After  opening  the  blow-off,  and  ex- 
pelling the  air  which  was  included  in  the  vulcanizer,  the  two  ther- 
mometers which  projected  into  the  interior  of  the  vulcanizer  agreed 
with  the  guage  and  table  of  pressures,  but  the  mercury  bath  ther- 
mometer was  still  in  error. 


TABLE  OF  STEAM  PRESSURES  AND  TEMPERATURES. 


Temp.     Lbs.  Pres. 

Temp.     Lbs.  Pres. 

Temp.     Lbs.  Pres. 

212   ...   0 

310  ...   62 

410  .  .  .  264 

220 

2 

320 

•   75 

420 

295 

230 

6 

330 

88 

430 

329 

240 

10 

340 

103 

440 

366 

2=0 

15 

350 

120 

450 

406 

260 

21 

360 

139 

460 

449 

270 

27 

370 

160 

470 

496 

280 

34 

380 

183 

480 

547 

290 

42 

390 

208 

490 

602 

300   . 

51 

400 

235 

500 

661 

t> 

1 

1 

1 

/ 

-160 
460 

n 

p^ 

•V 

«3 

71 

I 

1 

Nc 

>.1 

No.  2 

1 

3" 

-140 
130- 
-120 

-no 

-100 
-90- 

/ 

/Y^ 

~ ,  i 

1 

\ 

" 

/, 

\ 

/                y( 

1 

/ 

• 

,/' 

// 

/ 

-.,-...  ■. 

l^ 

/ 

z;' 

/ 

/ 

/ 

^ 

.-[/ 

/ 

^ 

• 

r 

• 

'/!._^ 

/ 

r 

• 
• 

^/\       /\ 

/ 

-^)^ 

No.  3 

! 

y' 

• 

A 

^ 

j 

, 

•" 

,• 

•' 

/ 

-30 

— 

^ " 

,^'' 

^-'^ 

y 

I 

' 

,^'- 

" 

.-'y^ 

'^> 

No.r'"- 

^ 

^ 

i^ 

'^ 

1         1     i 

200              220       1      2 

1                   1         i 

III 
0              2(50       j        2S0 

1 

300 

1 

1 

320 

340 

1 

1 

300 

.      L 

3S0 

Diagram  of  steam  pressures  and  temperatures  as  indicated  by  a  thermometer 
immersed  in  the  water  of  a  vulcanizer,  (No.  i,)  one  exposed  in  the  steam  space, 
♦(  No.  2,)  and  a  mercury  bath  thermometer,  (No.  3.)  Each  curve  is  numbered  in 
accordance  with  the  thermometer  from  whose  indications  it  is  made.  Those  made 
with  the  air  included  in  the  vulcanizer  are  formed  of  broken  lines;  with  the  air  ex- 
pelled, of  continuous  lines.  Nos.  i  and  2  are  identical  under  the  latter  condition, 
and  agree  with  the  table  of  the  elastic  force  of  steam,  given  above. 

The  results  obtained  are  most  easily  compared  and  studied  if 
represented  graphically,  and  the  above  diagram  has  accord- 
ingly been  prepared.  A  series  of  horizontal  lines  have  been  drawn, 
•each  representing  a  rise  in  pressure  of  ten  pounds,  as  compared 
with  the  line  below.  Another  series,  of  perpendicular  lines,  cross 
the  first;  each  representing  a  rise  in  temperature  of  ten  degrees, 
as  compared  with  the  line  to  its  left.  On  these  was  first  laid  off  a 
line,  No.  i,  to  correspond  with  the  table  of  temperatures  and  pres- 
sures. It  forms  a  curve,  and  it  will  be  noticed  at  once  how  much 
more  quickly  it  rises  as  the  temperature  and  pressure  increase,  the 


10 

same  increase  of  temperature  causing  a  much  more  rapid  rise  ot 
pressure  at  high  points  than  at  low  ones.  Other  lines  were  also 
drawn  as  noted  below  the  diagram. 

When  the  vulcanizer  was  heated  without  expelling  the  included 
air,  its  expansion  by  heat  operated  the  steam  gauge,  which 
showed  a  pressure  of  about  14  pounds,  at  212  degrees.  The  air 
pressure  is  gradually  overcome  by  that  of  the  steam,  and  in  the 
case  of  thermometer  No.  i,  the  line  gradually  merges  into  the 
normallines  of  pressures.  Thermometer  No.  2  shows  212  degrees 
at  20  pounds,  by  the  gauge,  and  its  curve,  though  it  gradually  ap- 
proaches the  normal  line,  does  not  touch  it  at  160  pounds.  A 
mixture  of  air  and  steam  is  not  a  good  conductor  of  heat;  but  its 
conducting  powers  increase  as  the  proportion  ot  .steam  in  it 
increases,  as  it  does  with  the  increasing  pressure. 

The  line  for  the  mercury  bath  thermometer  starts  at  a  still 
higher  figure,  27  pounds,  and  rises  more  rapidly  than  the  others, 
being  50  pounds  higher  than  the  normal  line  at  320  degrees;  show- 
ing the  temperature  to  be  really  350  degrees.  The  curves  illus- 
trating the  action  of  the  thermometers  under  the  influence  of 
included  air,    as  described  above,  are  shown  in  broken  lines. 

When  the  air  is  expelled,  both  No.  i  and  No.  2  thermometers 
reach  the  212  point  before  the  gauge  indicates  pressure,  and  follow 
the  normal  line.  No.  i,  closely  as  the  temperature  rises.  The 
mercury  bath  thermometer  reaches  the  212  point  when  the  gauge 
indicates  7  pounds,  and  its  line  diverges  from  the  normal  line  as  it 
rises;  so  that  at  the  320  point  the  pressure  is  98  pounds,  instead 
of  75,  as  in'  the  normal  line;  showing  that  it  fails  to  indicate  the 
true  temperature  by  about  15  degrees.  Therefore,  if  the  air  is 
expelled  from  the  vulcanizer,  the  temperature  is  really  320  degrees 
when  the  thermometer  indicates  305  degrees.  When  we  consider 
the  greater  rapidity  with  which  heat  is  abstracted  from  the 
exterior  of  the  vulcanizer  at  high  temperatures,  by  the  greater 
amount  of  radiation  and  by  convection  by  air  currents,  we  see  why 
the  error  of  the  mercury  bath  thermometer  increases  with  the 
temperature.  The  amount  of  this  error  differs  slightly  with  different 
vulcanizers,  according  to  their  construction. 

The  reason  for  the  apparent  variations  of  the  gas  regulator  is- 


11 

now  made  plain.  The  regulator  is  operated  by  pressure,  and  turns 
down  the  gas  when  a  certain  pressure  is  attained  in  the  vulcanizer. 
The  conditions  may  be  such  that  the  thermometer  fails  to  receive 
and  indicate  the  temperature  due  to  the  pressure;  in  which  case, 
the  reguhitor  is  right  and  the  thermometer  wrong.  When  using  a 
gas  regulator,  it  is  not  unusual  to  see  the  mercury  in  the  ther- 
mometer steal  upwards  ten  degrees  or  more  while  vulcanizing  is 
going  on  ;  for  as  the  included  air  escapes  through  the  leak,  the  ther. 
mometer  comes  nearer  to  registering  the  correct  temperature.  As 
the  presence  of  a  leak  necessitates  a  larger  flame  to  keep  up  the 
temperature,  the  valve  of  the  regulator  cannot  close  as  far  as 
though  the  vulcanizer  were  steam-tight,  and  the  temperature  of 
the  vulcanizer  will  be  lower,  acccording  to  the  size  of  the  leak. 
Whenever,  therefore,  the  regulator  fails  to  bring  the  vulcanizer  up 
to  the  usual  temperature,  a  leak  may  be  suspected,  and  should  be 
looked  for. 

It  may  be  laid  down  as  a  rule,  that  when  the  air  is  expelled 
from  the  vulcanizer,  the  actual  temperature  of  its  interior  is,  at 
the  vulcanizing  point,  some  15  deg.  more  than  is  indicated  by  the 
mercury  bath  thermometer,  while  if  the  air  is  not  expelled,  it  may  be 
as  much  as  35  deg.  higher.  Vulcanizing  may  therefore  be  done  at  355 
when  the  dentist  has  no  idea  that  320  has  been  exceeded.  Bear- 
ing this  fact  in  mind,  the  reason  for  occasional  mysterious  appear- 
ances of  spongy  rubber  is  obvious,  and  the  remedy  apparent. 
The  difficulty  can  be  avoided  by  attention  to  the  condition  of  the 
vulcanizer,  and  possibly,  lowering  the  temperature. 

Experience  will  show  beyond  question  that  the  best  work  is 
done  by  the  use  of  a  low  temperature  and  longer  time. 

Steam  gauges  have  been  recommended^  as  substitute  for  the 
thermometer.  Like  the  gas  regulator,  the  steam  gauge  operates 
by  pressure,  and  the  pressure  due  to  even  the  slightest  increment 
of  heat  is  immediately  felt  throughout  the  interior  of  the  vulcan- 
izer, while  the  temperature  is,  as  we  have  seen,  subject  to  delays 
in  its  passage  upwards  to  the  thermometer.  The  steam  gauge  is 
therefore  the  more  sensitive  instrument,  but  it  is  also  more  expen- 
sive, more  complicated,  and  more  likely  to  get  out  of  order  than 
the  thermometer,  which  from  its  low  cost,  simplicity  and  durability. 


12 

has  enjoyed  and  will  continue  to  enjoy  the  preference  as  a  means 
for  ascertaining  the  temperature  of  the  vulcanizer. 

The  gas  regulator  has  been  described  as  operating  by  steam 
pressure.  It  is  possible  to  make  an  instrument  of  the  kind  which 
will  operate  by  expansion  of  a  metal  by  heat,  and  such  instruments 
have  been  made. 

When  the  subject  is  properly  understood,  however,  it  will  be 
obvious  that  preference  is  to  be  given  to  those  operated  by  steam 
pressure,  on  account  of  their  greater  sensitiveness  as  the  tem- 
perature increases,  if  for  no  other  reason;  there  being,  moreover, 
no  saving  as  regards  simplicity  of  construction  in  the  one  pattern 
of  instrument  over  the  other. 


THE  PHILOSOPHY  OF  THE 
VULCANIZATION  OF  RUBBER, 

AS    APPLIED    TO    DKNTAL    PLATES. 

\J[    MIXTURE  of  India   rubber  and  sulphur   in   certain   propor- 
tions,  will,  when  subjected  to  heat  and  pressure,  undergo  a 
molecular  change   which  is  known  as  vulcanization.      The  nature 
of  this  change  is  not  clearly  understood,  but  it  is  known  to  be  ac- 
companied by; 

1.  An  increase  in  the  hardness  of  the  mass;  which,  from 
being  soft  and  plastic,  becomes  hard  and  horny  in  texture. 

2.  An  increase  in  specific  gravity;  the  mass  becoming  smaller 
in  proportion  to  its  weight. 

3.  A  decrease  in  susceptibility  to  atmospheric  influences,  and 
the  action  of  solvents;  there  being  but  few  substances  which  will 
affect  vulcanized  rubber,  while  there  are  many  which  will  attack 
the  unvulcanized  gum. 

'I'iie  second  condition  mentioned  above  has  not  received  the 
attention  it  should  from  dentists,  for  it  is  responsible  for  some  very 
grave  annoyances  experienced  l)oth  by  the  dentist  and  his  patient. 

The  specific  gravity  of  a  piece  of  unvulcanized  rubber  compound, 
(black)  was  found  to  be  1.1333-  The  same  piece,  vulcanized, 
had  a  specific  gravity  of    1.1974.      Tiiis   would   show  a   diminution 


13 

of  bulk  by  vulcanizing  of  about  five  per  cent.  Different  samples 
will  vary  to  some  extent  in  their  specific  gravity,  and  possibly  in 
their  behavior  in  vulcanizing;  but  it  is  a  well-established  fact  that 
Ai.L  RUHHERS SHRINK  IN  VULCANIZING;  the  purer  the  sample  the  great- 
er being  the  shrinkage.  Those  rubbers  which  are  colored  by  the  ad- 
mixture of  pigments,  e.  g.,  vermillion,  oxide  of  zinc  etc.,  shrink 
less  just  in  proportion  to  the  amount  of  foreign  matter  which  they 
contain.  The  shrinkage  is  less,  but  it  unfortunately  happens  that 
this  benefit  is  accompanied  by  a  loss  of  tenacity.  Rubbers  which 
are  loaded  with  earthy  or  coloring  matters  are  not  so  strong  as 
those  which  are  not  so  treated. 

Experiments  have  developed  the  fact  that  samples  of  rubber 
vulcanized  at  high  temperatures,  and  consequently  under  high 
steam-pressure,  have  a  greater  specific  gravity  than  those  vulcan- 
ized at  a  low  temperature  and  pressure. 

The  amount  of  shrinkage  which  rubber  experiences  in  vul- 
canizing therefore  varies  to  some  extent,  and  depends  upon  the 
temperature  of  the  vulcanizer  as  well  as  the  time  of  vulcanization. 

Rubber  solidifies  more  rapidly  in  the  early  part  of  the  vulcan- 
izing process;  the  change  in  density  and  specific  gravity  being 
nearly  completed  during  the  first  two-thirds  of  the  time  required 
to  properly  harden  the  mass. 

Rubber  expands  by  heat  more  rapidly  than  any  other  solid 
body.  Its  rate  of  expansion  at  ordinary  temperatures,  from  70  to 
90  Fahr.  is  over  six  times  that  of  iron,  about  five  times  that  of 
brass,  and  nearly  four  times  that  of  zinc,  which  is  the  most  suscep- 
tible to  expansion  by  heat  of  any  of  the  metals.  Its  rate  of 
expansion  is  known  to  increase  as  the  temperature  rises,  but  it  has 
not  been  definitely  ascertained. 

When  it  is  desired  to  construct  a  dental  plate  of  rubber,  a 
mould  is  prepared,  separable  into  two  halves,  of  which  the  teeth 
and  a  model  of  the  mouth  form  component  parts.  Gateways,  to 
allow  of  the  escape  of  any  excess  of  rubber,  are  cut  in  the  parting 
surfaces  of  the  mould,  which  is  then  packed  with  rubber  com- 
poud.  The  two  halves  of  the  mould  are  then  brought  together  by 
means  of  bolts;  heat  being  applied  to  soften  the  rubber,  which 
yielding  gradually  to  the  pressure  of  the  bolts,  flows  into  and  ob- 


14 

literates  the  vacancies  left  by  careless  packing,  any  excess  escaping 
into  the  gateways.  When  the  flask  is  completely  closed,  it  is 
placed  in  the  vulcanizer. 

As  the  temperature  rises,  the  rubber  expands,  and  until  the 
vulcanizing  point  is  reached,  there  is  a  constant  flow  of  rubber  into 
the  gateways.  When  the  desired  heat  is  attained,  and  the  tem- 
perature becomes  stationary,  the  mass  begins  to  solidify.  It  now 
gains  in  specific  gravity  and  shrinks,  and  in  a  few  moments  it  is  no 
longer  capable  of  filling  the  mould.  As  the  rubber  coheres  to  the 
plaster  more  strongly  than  it  does  to  porcelain,  it  is  usually  the 
case  that  no  defects  from  shrinkage  are  apparent  on  the  surface 
of  the  plates,  but  if  teeth  are  removed,  vacancies  will  be  found 
under  them,  and  the  platinum  pins  will  be  found  loose  in  the  rub- 
ber. 

Sometimes,  though  not  often,  the  effects  of  shrinkage  are  seen 
as  small  depressions,  like  the  impression  of  a  split  pea,  occurring 
indifferently  on  the  lingual  or  palatal  sides  of  the  plate.  More 
often,  nothing  is  noticed  until  it  becomes  necessary  to  remove  some 
of  the  teeth  from  the  plates;  though  when  plain  teeth  are  used, 
spaces  are  sometimes  found  under  the  shoulders  of  the  molars  and 
bicuspids.  If  a  tooth  stands  alone,  as  in  a  partial  plate,  it  will 
sometimes  be  found  to  be  loose,  shaking  perceptibly  when  an  at- 
tempt is  made  to  move  it. 

The  fact  that  the  teeth  and  pins  are  loosely  held  in  the  rubber 
can  be  ascertained  by  rattling  the  plate  on  a  table  or  counter,  as  a 
coin  is  tested.  It  will  be  found  to  give  out  a  sound  as  though  it 
were  cracked. 

The  vulcanizing  process  being  concluded,  the  plate  cools 
down  to  the  temperature  of  the  room,  and  in  doing  so  contracts. 
As  will  be  explained  later  on,  this  contraction  is  a  fruitful  source 
of  warping  and  misfits.      Vov  the  ]:)resent  we  have  to  consider 


SHRINKAGE,  AND   HOW  TO  OVERCOME  IT. 

A  careful  inspection  of  old  plates  when  breaking  them  up,  will 
show  it  to  be  the  rule,  rather  than  the  exception,  for  the  pins  to  be 
more  or  less  hjose  in  the  rubber;  and  when  section  teeth  have  been 


15 

used,  and  set  at  a  distance  from  the  model  so  as  to  allow  of  a  con- 
siderable thickness  of  rubber  between  the  teeth  and  the  plate, 
spaces  will  be  found  under  the  teeth,  which,  if  the  plate  has  been 
worn  for  any  length  of  time,  will  be  filled  with  a  magma  of  putre- 
fying food.  This  is  the  cause  of  the  disgusting  odor  which  taints 
the  breath  of  the  wearer  of  the  plate  to  his  annoyance  and  that  of 
all  who  have  occasion  to  come  near  him. 

To  overcome  this  defect,  enough  material  must  be  retained  in 
the  mould  to  insure  its  being  perfectly  filled  at  the  conclusion  of 
the  vulcanizing  process.  This  may  be  accomplished  by  interpos- 
ing thin  slips  of  metal,  (e.  g. ,  heavy  tin  foil),  between  the  two 
halves  of  the  flask,  so  that  it  will  not  be  fully  closed.  Then,  after 
the  plate  is  about  half  vulcanized,  the  flask  should  be  removed  from 
the  vulcanizer,  the  strips  of  metal  removed,  and  pressure  applied 
to  complete  the  closing  of  the  flask.  The  rubber  will  be  too  hard 
to  yield  to  the  pressure  of  bolts  as  usually  applied,  and  it  is  neces- 
sary to  use  spring  pressure,  which  can  be  continued  after  the  flask 
is  replaced  in  the  vulcanizer,  the  rubber  being  still  soft  enough  at 
the  vulcanizing  temperature  to  flow  and  accommodate  itself  to  the 
mould.  The  second  heating  must  immediately  follow  the  first,  as 
the  plaster  soon  disintegrates  and  softens  and  will  not  then  bear 
the  pressure  required  to  re-mould  the  rubber. 

Some  judgment  is  required  as  to  the  thickness  of  the  inter- 
posed slips,  and  the  amount  of  spring  pressure  to  be  applied.  For 
an  upper  plate  with,  say  a  sixteenth  of  an  inch  of  rubber  between 
the  teeth  and  model,  slips  of  about  the  thickness  of  No.  30  plate 
will  answer;  for  very  light  cases,  even  thinner  ones  may  be  used; 
for  heavy  lower  plates  slips  as   thick  as  No.  24  may  be  necessary. 

In  gating  the  flask, remove  all  the  surface  of  the  plaster  except 
a  very  narrow  margin  immediately  around  the  mould  by  a  cut 
gradually  deepening  as  it  passes  outwards.  Cut  no  notches  from 
this  channel  into  the  mould  itself,  as  the  springs  will  yield  to  the 
pressure  of  the  rubber  as  it  expands,  and  allow  the  two  halves  of 
the  flask  to  open  sufficiently  to  allow  the  surplus  to  escape. 

The  springs  supplied  with  the  flasks  of  the  New  Model  Whit- 
ney Vulcanizer  require  a  force  of  from  iSo  to  200  lbs.  to  fully 
close  them.      They  are  therefore  capable  of  putting  a  pressure  of 


16 

from  540  to  600  lbs.  upon  the  flask.  This  amount  is  not  necessary 
or  advisable  except  when  a  large  surface  of  rubber  is  exposed,  and 
if  the  plate  is  small  the  springs  should  be  only  partially  com- 
pressed. 

As  the  teeth  are  sometimes  cracked  by  over-pressure,  care 
should  be  taken  not  to  bring  the  coils  of  the  springs  into  absolute 
contact,  and  throw  them  out  of  action. 

It  appears  that  the  expansion  of  the  rubber  compound  in  heat- 
ing from  212  degs.  to  the  vulcanizing  point  is  about  equal  to  its 
shrinkage  in  vulcanizing.  It  is  possible,  therefore,  to  dispense 
with  the  interposition  of  the  thin  slips  between  the  halves  of  the 
flask,  by  simply  relieving  all  pressure  thereupon  during  the  first 
part  of  the  time  of  vulcanizing  and  allowing  the  flask  to  open  as 
the  rubber  expands.  Spring  pressure  must  then  be  applied,  after 
shrinkage  is  practically  completed,  to  close  the  flask  as  before. 

The  disadvantage  of  this  method  is  that  as  the  pressure  upon 
the  rubber  is  entirely  relieved,  it  will  not  flow  and  obliterate  the 
small  imperfections  which  sometimes  occur  in  packing  the  mould. 
If  it  is  to  be  employed,  then  the  rubber  compound  must  be  thor- 
oughly softened,  and  especial  care  taken  to  pack  it  so  that  there 
shall  be  no  faults,  employing  something  of  the  same  degree  of  care 
that  would  be  exercised  in  filling  a  tooth.  Spring  pressure  is  then 
to  be  applied  to  close  the  flask,  it  being  boiled  in  a  dish  of  water 
until  its  closure  is  complete.  The  nuts  are  then  slackened  to  re- 
lieve all  pressure  upon  the  springs,  and  the  flask  placed  in  the  vul- 
canizer. 

The  processes  of  vulcanizing  above  described  form  the  subject 
of  Letters  Patent,  granted  to  the  undersigned  on  June  16,  1S91. 
Ofifice  rights  to  use  the  same  can  be  obtained  by  application  to 

GEORGE   B.   SNOW, 

Bufi^alo.  N.  Y. 


17 
SPONGY  RUBBER. 

^.C)  HEN  thick  pieces  of  Rubber  are  vulcanized,  it  is  sometimes 
vV  the  case  that  if  they  are  cut  into,  they  will  be  found  not  to 
be  hardened  entirely  through;  the  interior  Ijeing  soft,  spongy,  and 
fetid. 

This  occurs  more  fre(}uently  when  black  rubber  is  used,  and 
less  so  with  those  containing  much  foreign  matter,  the  scale  run- 
ning from  black,  whalebone,  red,  light  red,  to  pink  and  white;  the 
latter  seldom,  if  ever,  becoming  spongy  under  ordinary  treatment. 
The  purer  the  rubber,  then,  the  more  apt  it  is  to  become  spongy, 
and  the  more  it  is  adulterated,  the  thicker  the  mass  of  it  which 
can  be  safe.ly  vulcanized. 

Sponginess  is  more  liable  to  occur  as  the  vulcanizing  temper- 
ature is  raised;  and  as  this  may  happen  by  some  derangement  of 
the  thermometer,  this  instrument  should  be  carefully  looked  to  if 
there  is  any  trouble  of  the  kind.  Thick  pieces  cannot  be  cured  at 
as  high  a  heat  as  thinner  ones;  and  when  it  becomes  necessary  to 
vulcanize  one,  especial  care  must  be  taken  that  the  heat  does  not 
run  too  high.  When  sponginess  does  occur,  unless  the  piece  is  un- 
usually thick,  it  may  be  at  once  assumed  that  the  heat  has  been 
too  great;  and  in  subsequent  operations  it  should  be  lowered  with- 
out any  regard  to  what  the  indication  of  the  thermometer  may  be, 
the  time  of  vulcanizing  being  accordingly  increased. 

When  it  becomes  necessary  to  vulcanize  an  unusually  thick 
piece,  its  interior  may  be  packed  with  pink,  or  some  other  heavily 
"loaded"  rubber;  or  pieces  of  old  plates,  cut  into  convenient 
sizes  and  filed  all  over,  so  as  to  present  clean  surfaces,  may  be 
mixed  with  'the  fresh  rubber.  The  latter  plan  is  preferable  if  the 
piece  is  very  thick,  as  it  also  diminishes  the  amount  of  shrinkage 
in  vulcanizing.  If  the  pieces  are  clean,  with  fresh  surfaces,  they 
will  unite  perfectly  with  the  new  material,  and  there  will  be  no 
lack  of  strength  in  the  work  when  finished.  The  more  old  rubber 
used,  the  less  shrinkage  there  will  be;  as  it,  being  already  vul- 
canized, has  shrunk  nearly  all  it  is  capable  of  doing. 

As  there  will  be  a  difference  of  color  between  the  new  and  the 
old  rubber,  the  latter  should  not  be  allowed  to  come  to  the  sur- 
face, if  it  can  be  prevented. 


18 

Sponginess  may  occur  as  an  accident,  from  a  sudden  relief  of 
pressure;  as,  for  instance,  from  the  blowing  out  of  a  safety  disk. 
It  is  particularly  apt  to  happen,  if  the  disk  is  blown  out  during  the 
first  half  hour  of  the  time  of  vulcanizing;  the  rubber  being  then 
soft,  and  changing  in  consistency  more  rapidly  than  thereafter. 
The  fact  of  the  failure  of  the  disk,  is  a  pretty  sure  indication  that 
the  temperature  has  been  inordinately  high,  and  sponginess  might 
occur  from  this  cause  alone,  without  any  relief  of  pressure.  But 
it  wdl  take  place  if  the  blow-off  valve  is  opened  during  the  first 
stages  of  the  vulcanizing  process,  if  the  piece  is  at  all  thick  and 
enough  steam  is  allowed  to  escape  to  lower  the  pressure  to  any 
great  extent. 

When  a  disk  blows  out,  the  escape  of  steam  can  be  stopped 
by  driving  a  plug  of  soft  pine  into  the  hole  from  which  it  escapes. 
It  is  well  to  keep  one  or  two  plugs  ready  prepared,  as  the  lower- 
ing of  pressure  in  the  vulcanizer  consequent  upon  a  few  moments' 
delay  may  cause  the  rubber  to  become  spongy  and  ruin  the  plate. 


WARPING  OF  PLATES. 

^  I.THOUGJI  rubber  plates  are  moulded  and  hardened  upon 
^"-^^  the  models  of  the  mouths  they  are  to  fit,  they  often  fail  to 
fit  as  they  should.  The  trouble  may  arise  from  a  faulty  im- 
pression, or  from  carelessness  in  running  the  model.  There  are 
some  who  are  ignorant  of  the  fact  that  the  model  may  be  warped 
by  delay  in  pouring  the  plaster  into  the  impression.  If  a  little  is 
poured  in,  then  the  remainder  is  stirred  before  pouring  it,  and  this 
isrepeated  until  the  impression  is  finally  filled,  the  model  will  arch  up 
so  that  its  palatine  portion  wi-ll  leave  the  impression,  and  a  space 
will  be  clearly  seen  between  the  two.  The  plaster  should  always 
be  mixed  thin  enough  to  run  freely,  and  the  impression  filled  as 
speedily  as  possible. 

It  has  been  heretofore  iKHed  that  iiibber  is  probably  the  most 
sensitive  to  fluctuations  of  tem[)erature  of  any  solid  body.  As  the 
difference  between  the  vulcanizing  temperature,  at  which  plates 
arc    inoulded,  and    that    of    the    mouths  in  which  they  are   worn, 


19 

amounts  to  considerably  over  two  luuidred  degrees,  the  contrac- 
tion of  the  plate  is  quite  perceptible;  still  it  does  not  interfere  with 
the  fit  unless  gum  teeth  are  used.  Porcelain,  of  which  the  teeth 
are  made,  changes  but  little  with  fluctuations  of  temperature;  and 
when  an  arch  of  it  is  formed  by  lateral  contact  of  the  section 
teeth,  it  renders  more  apparent  the  contraction  of  the  rubber.  If 
careful  measurements  be  taken  across  the  condyles  of  a  plate 
mounted  with  section  teeth  before  and  after  vulcanizing,  the  sec- 
ond measurement  will  be  found  to  be  considerably  shorter  than 
the  first;  the  condyles  having  been  brought  together  by  the  con- 
traction of  the  rubber  in  cooling.  As  a  consequence, the  thin,  palatal 
portion  of  the  plate  is  forced  upwards,  and  when  it  is  tried  in  the 
mouth  it  will  be  found  to  rock.  If  this  defect  should  not  be 
noticed,  a  sore  spot  upon  the  hard  palate  will  usually  be  developed 
after  a  few  days  wear. 

If  the  plate  is  a  new  one,  the  defect  can  be  usually  remedied 
by  warming  the  palatal  edge  of  the  plate  sufficiently  to  soften  the 
rubber,  and  bending  it  downwards  enough  to  relieve  the  pressure, 
it  being  seldom  the  case  that  there  will  be  any  bad  effect  from  the 
narrowing  at  the  condyles.  But  when  an  old  plate  is  re-vulcan- 
ized, its  contraction  is  thereby  doubled;  and  it  often  happens  that 
there  will  be  a  serious  impairment  of  the  fit. 

To  restore  the  fit  of  the  plate,  it  should  be  spread  or  widened 
at  the  condyles  to  compensate  for  its  previous  contraction.  Put  a 
small  mark  upon  each  condyle  and  measure  the  distance  betwcr  i 
them.  This  can  be  most  easily  done  with  a  pair  of  dividers. 
Then  direct  light  puffs  of  a  blow-pipe  fiame  upon  the  plate, 
directly  back  of  the  central  incisors  until  it  is  softened  throughout 
its  thickness  for  a  space  about  as  large  as  the  thumb  nail.  Seize  it 
by  the  condyles  with  both  hands,  pull  them  forcibly  apart,  and 
at  the  same  time  dip  the  plate  into  water  to  cool  it.  The  dividers 
will  show  the  amount  of  alteration  produced;  if  it  should  be  too 
much,  a  slight  re-softening  of  the  plate,  as  before,  will  allow  it  to 
partially  regain  its  former  shape. 

When  a  plate  is  to  be  re-vulcanized,  it  will  be  well  to  treat  it 
in  this  manner  before  putting  it  in  the  vulcanizer;  widening  it  at 
the  condyles  nearly  a  sixteenth  of  an  inch,  thus  compensating  for 


20 

the  former  shrinkage  and  warping  as  well   as   the  one  from  the  in- 
tended vulcanizino-. 


CRACKING    OF  BLOCKS. 

rnHE  Dentist  is  sometimes  dismayed  by  finding  an  otherwise  sat- 
I  isfactory  piece  of  plate  work  marred  by  the  appearance  of 
cracks  in  the  teeth.  The  plate  may  have  appeared  to  be  perfect 
when  first  taken  from  the  vulcanizer,  and  the  cracks  may  not  be 
noticed  until  it  is  nearly  or  quite  finished.  As  it  oftentimes  neces- 
sitates making  a  new  plate,  the  causes  which  lead  to  this  very 
annoying  accident  are  deserving  of  careful  study.  They  may  be 
divided  into  three  classes,  viz.  : 

1.  Excessive  pressure  upon  the  teeth  in  packing  the  mould; 
which  maybe  caused  by: 

a.  The  presence  of  a  large  excess  of  rubber;  filling  and 
clogging  the  gateways. 

b.  Attempting  to  close  the  flask  before  the  included  rubber  is 
thoroughly  warmed  and  softened. 

c.  Using  undue  force  in  closing  the  flask,  and  not  allowing 
the  rubber  time  to  flow. 

2.  The  effects  of  contraction  of  the  rubber  in  cooling. 

3.  Carelessness  in  handling  the  plate  after  it  is  taken  from 
the  vulcanizer. 

Rubber  compound,  even  when  warmed,  is  very  viscid;  and 
while  it  is  quite  capable  of  intruding  into  places  where  it  is  not 
wanted,  as,  for  instance,  the  joints  between  section  teeth,  it  offers 
considerable  resistance  to  any  suddenly  applied  force.  The  oper- 
ator should,  therefore,  never  be  in  a  hurry  in  closing  his  flasks; 
but  apply  the  pressure  gradually,  and  allow  plenty  of  time  for  the 
rubber  to  flow.  No  attempt  should  be  made  to  close  the  flask 
until  after  it  has  been  thoroughly  warmed,  and  plenty  of  time 
given  for  the  heat  to  penetrate  and  soften  the  rubber. 

Over-packing  the  mould,  by  obstruction  of  the  gateways  by 
the  inordinate  amount  of  surplus,  may  lead  to  the  application  of 
too  much  force  in  closing  the  flasks;  and,  as  a  consequence,  sec- 
tion teeth  may  be  cracked. 


21 

If  the  gateways  are  insufficient,  or  are  clogged  by  the  escap- 
ing surplus,  and  the  flask,  when  closed,  is  held  rigidly  by  bolts, 
the  expansion  of  the  rubber  under  the  rise  of  temperature  to  the 
vulcanizing  point  may  exert  a  force  quite  sufficient  to  do  damage 
to  the  teeth.  If  the  flask  is  held  under  spring  pressure,  it  will  be 
able  to  yield  to  the  expansive  force,  and  allow  the  escape  of  the 
surplus;  then,  if  the  flask  should  remain  a  trifle  open,  the  springs 
will  continue  their  pressure  and  close  it  when  the  shrinkage  of  the 
rubber  in  vulcanizing  relieves  the  internal  pressure.  Indeed,  the 
flask  and  its  contents  may  be  safely  trusted  to  the  springs,  even 
though  it  be  not  fully  closed,  if  there  is  a  certainty  that  the  gate- 
ways are  free.  The  gateway  may,  in  this  case,  be  only  a  deep, 
wide  groove,  with  a  narrow  margin  of  plaster  left  between  it  and 
the  mould;  no  "runners,"  or  passages  leading  outward  from  the 
mould  being  necessary.  This  method  of  gating  the  flask  was  advo- 
cated by  Dr.  W.  S.  How  in  an  article  in  the  "Dental  Cosmos," 
for  July,  1889,  and  has  advantages  which  should  lead  to  its  adop- 
tion. 

It  has  been  before  noted  that  rubber  is  the  most  sensitive  to 
the  changes  of  temperature  of  any  solid  body.  Its  contraction 
in  cooling  from  the  vulcanizing  point  to  ordinary  temperatures  is 
sufficient  in  amount  to  cause  considerable  warping  of  the  plate 
when  section  teeth  are  used;  and,  in  some  instances,  the  pull  of 
the  rubber  upon  the  sections  is  strong  enough  to  cause  vertical 
checks  to  appear  in  the  centers  of  the  front  sections.  It  is  also 
the  case  that,  although  the  plate  may  safely  bear  the  reduction  of 
temperature  to,  say,  60  or  70  degrees,  the  temperature  of  the 
office,  it  will  not  bear  a  further  reduction  to  the  freezing  point  or 
below.  If,  therefore,  the  laboratory  is  left  without  a  fire  at  times 
in  cold  weather,  the  dentist  may  be  disagreeably  surprised  to  find 
the  piece  of  work  which,  he  left  in  a  perfect  condition  Saturday 
night,  with  the  front  teeth  checked,  when  he  examines  it  on  the 
ensuing  Monday.  Patients  should  be  cautioned  against  leaving 
unused  plates  where  they  will  be  exposed  to  extreme  cold,  or 
dipping  their  plates  into  very  cold  water.  Though  no  harm  may 
result,  there  is  a  risk  of  damage  from  such  practices  which  it  is 
better  to  avoid 


22 

Improper  grinding  of  the  teeth,  taken  in  conjunction  with  the 
shrinkage  of  the  rubber,  is  a  fruitful  source  of  annoyance  from 
cracking  and  chipping.  If  the  ends  of  the  sections  are  ground 
away  too  much  at  the  rear, so  that  only  a  narrow  surface  is  left  for 
contact  immediately  at  the  gum  surface,  the  pressure  on  the  same 
from  contraction  is  often  sufficient  to  chip  the  edges  of  the  joint. 
Though  a  bearing  over  the  whole  end  of  the  block  is  unneces- 
sary, it  should  be  of  an  appreciable  width. 

If  the  rim  of  the  plate  is  thick,  and  the  edges  of  the  gums 
are  left  so  that  they  are  held  by  it,  its  contraction  will  sometimes 
exert  force  enough  upon  the  gums  to  crack  them.  In  this  case, 
the  cracks  will  proceed  horizontally  along  the  ^ms,  just  above 
the  bases  of  the  teeth,  and  will  be  clean  ;  no  rubber  being  in- 
jected into  them.  This  state  of  affairs  is  sometimes  brought 
about  by  grinding  the  gums  thin  in  fitting  the  teeth  to  the  model, 
whfjn  there  was  no  necessity  for  so  doing. 

The  rubber  is  drawn  away  from  the  rear  of  the  gums  by  its 
contraction,  and  affords  them  no  support.  The  edges  of  the 
gums  should  therefore  be  ground  so  that  the  rubber  will  have  no 
hold  upon  them.  They  should  be  ground  to  a  square  edge,  or  if 
either  corner  of  the  edge  is  left  sharp,  it  should  be  the  front, 
rather  than  the  rear  one.  In  waxing  up,  especial  care  should  al- 
ways be  taken  that  the  wax  does  not  overhang  the  g.ums.  It 
should  be  scraped  off  so  as  to  be  exactly  flush  with  the  gum:  the 
necessary  surplus  for  filing  and  finishing  being  secured  by  scraping 
off  a  little  plaster  from  the  proper  locality  after  the  case  is  llasked, 
and  before  packing. 

In  the  matter  of  careless  handling  of  the  plate,  there  is  but 
little  to  be  said.  Damage  maybe  d(jne  in  gettmg  them  out  of 
the  (Task,  or  by  grasping  them  too  hard  when  finishing  thcni. 
They  should  never  be  pinched,  so  as  to  force  the  condyles  to- 
gether, as  this  intensifies  the  strain  the  plate  is  already  under  from 
contraction  of  the  rubber. 


THE    CONTOUR    OF    THE    LINGUAL   SURFACE    OF 
THE  DENTAL  PLATE. 

niHE  best  results  in  art  come  to  him  who  makes  the  closest  study 
I  of  nature.  One  instance  of  the  truth  of  this  saying  is 
evinced  by  the  marvelous  advance  made  in  the  appearance  of  arti- 
ficial teeth,  and  their  adaptation  to  the  mouth.  It  is  sometimes  the 
case,  though,  that  after  the  dentist  has  selected  teeth  (jf  the  cor- 
rect shade,  shape,  and  size,  and  has  mounted  them  upon  a  satis- 
factorily fitting  plate,  the  wearer  still  loses  something  of  the  com- 
fort and  satisfaction  he  might  have,  by  reason  of  improper  con- 
touring of  the  plate  on  its  lingual  surface. 

An  excellent  object  lesson,  one  that  dentists  unfortunately 
too  seldom  avail  themselves  of,  may  be  had  by  a  careful  inspec- 
tion and  study  of  plaster  casts  of  mouths  containing  perfect  Sets 
of  natural  teeth.  If  these  casts  are  divided  by  a  vertical  section 
through  the  mesial  line,  the  sectional  curve,  formed  by  the  lingual 
surfaces  of  the  incisors,  the  gum  and  the  palate,  is  worthy  of 
especial  attention.  As  a  contrast,  a  similar  cast,  taken  from  a 
"job"  on  rubber,  similarly  divided,  may  also  be  studied.  To  one 
whose  attention  has  never  been  directed  to  the  matter,  the  dif- 
ference between  the  curves  of  the  mouth  and  the  plate  will  be  a 
source  of  astonishment.  In  the  mouth  there  is  a  gradual  easy 
curve  from  the  point  of  the  incisor  to  the  palate.  In  the  plate  the 
material  is  usually  cut  away  behind  the  incisors,  making  two 
curves  instead  of  one,  and  these  joined  by  an  abrupt  angle  at  the 
rear  of  the  lingual  surfaces  of  the  incisors. 
■  What  are  the  objections  to  this  practice? 

A  considerable  portion  of  the  strength  of  the  plate  is  sacri- 
ficed, and  in  some  cases  it  splits  through  the  middle,  when  worn, 
from  no  other  cause  than  the  manner  in  which  it  has  been  cut  away 
just  behind  the  incisors. 

The  proper  enunciation  of  the  "S"and  "Sh"  sounds  is  only  to 
be  secured  by  making  the  contour  of  the  lingual  surface  of  the  plate 
to  resemble  that  of  the  mouth.  These  sounds  are  formed  between 
the  tongue,  the  lingual  surfaces  of  the  incisors,  and  the  gum  and 


24 

palate  behind  the  incisors.  The  whistling  or  whirring  S  sound,  b}' 
which  it  is  so  easy  to  detect  the  wearer  of  a  dental  plate,  is  di- 
rectly attributable  to  the  incorrect  contour  of  the  surface  in  ques- 
tion. 

It  is  now  proposed  to  describe  the  mechanism  b)'  which  these 
sounds  are  produced,  and  to  draw  attention  to  the  importance  of 
giving  due  consideration  to  the  shape  of  the  lingual  surface  of  the 
plate  in  order  to  secure  for  its  wearer  clearness  and  ease  of  articu- 
lation of  the  sounds  referred  to. 


Fig.  I. 


Fig.  2. 


Figure  i  is  a  diagram  illustrating  the  relative  positions  of  the 
jaws  and  tongue  in  producing  the  "S"  sound.  The  teeth  are  held 
slightly  apart,  the  tip  of  the  tongue  rests  against  the  gum  behind 
t"he  upper  incisors,  the  edges  impinging  closely  against  the  lingual 
surfaces  of  the  bicuspids  and  molars  at  the  junction  with  the  al- 
veolus. A  narrow  passage  is  thus  formed  over  the  center  of  the 
tongue,  through  which  a  current  of  air  is  impelled  against  the  in- 
cisors. 

In  making  the  "Sh"  sound  (Fig.  2)  the  tongue  is  pushed  a  little 
farther  forwards,  and  its  tip  is  directed  downwards.  The  passage 
over  its  center  has  a  larger  outlet,  and  the  current  of  air  is  directed 
against  the  lower  incisors. 

Attention  is  drawn  to  the  easy  reversed  curve  formed  by  the 
teeth  and  gum,  and  to  the  manner  in  which,  in  connection  with  the 
tongue,  a  passage  is  produced  which  gradually  narrows  and  then 
widens,  and  by  which  the  sounds  in  question  are  readily  and  clear- 
ly produced. 


25 

Figure  3  represents  a  section  of  a  fairly  well  made  vul- 
canite plate.  The  lingual  surface  of  the  incisor  here  forms 
an  angle    with    the    palatal    surface    of   the  plate,  and    the    re- 


Fig.  ?■  Fig.  4. 

versed  curve  of  Fig's,  i  and  2  is  plainly  out  of  the  question.  To 
obtain  this,  the  plate  should  be  filled  in  as  shown  in  dotted  lines  in 
Fig.  4.  If  this  were  done,  the  enunciation  of  the  wearer  would  be 
improved,  and  another  substantial  benefit  would  be  secured;  an 
increase  of  strength  which  would  obviate  any  ganger  of  the  plate 
cracking  through  the  center.  This  very  annoying  accident  is  very 
often  the  result  ot  excessive  cutting  away  of  the  thickness  of  the 
plate  at  the  point  in  question.  As  the  end  of  the  gum  section  is 
plainly  shown  in  either  figure,  it  can  be  seen  at  a  glance  how  little 
rubber  there  is  behind  it  in  the  one  case,  and  how  much  stronger 
it  would  be  in  the  other  for  the  change  suggested. 

At  first  thought,  it  appears  to  be  the  proper  thing  to  "make 
the  plate  light",  and  the  filling  up  which  is  necessary  to  restore 
the  right  contour  looks  bungling.  A  fair  trial,  in  even  a  few 
cases,  will  convince  anyone  that  the  latter  plan  is  the  right  one; 
i.  e.  to  fill  in  behind  the  incisors  in  waxing  up,  so  that  the  surface 
of  the  plate  will  form  an  easy  ''reversed  curve,"  beginning  with 
the  lingual  surfaces  of  the  incisors,  and  extending  backwards 
nearly  or  quite  half  an  inch  over  the  plate  before  the  normal 
thickness  of  the  plate  is  reached. 


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